Discussion
Studies of sensitization rate to pollen have been insufficient because of the lower allergenicity of pollen than that of house dust mites. Further, only a few studies have investigated sensitization in childhood. The present study is to investigate the change of sensitization rates to pollen in children with allergic diseases taken allergy test with altered pollen season for last 22 years. The present outcomes have important implications for the use of phenology as an indicator of meteorological changes. A shift in the timing of the common tree pollen season is important as known pollens also may aggravate the symptoms of respiratory allergic diseases. The outcome might be implied to increase the sensitization rate to tree pollen and pollen allergy from year to year for the past 22 years, and the age of sensitization rate to common tree pollens was also extended to younger children under 10 age of year.
Allergic rhinitis and allergic conjunctivitis have been presented over teen aged children9 but a number of pediatrician and allergists have reported a gradual increase in the prevalence of allergic diseases in children aged <6 years.17 The present study focused on the allergic children with age 3~18 years old at Seoul metropolitan area for the last 22 years.
As pollen exposure rates differ depending on annual weather, sensitization rates were characterized according to these differences in the present study. There may be implied 3 primary means by which weather change could influence allergic sensitization to pollen.
First, warmer temperature due to climate change might increase seasonal intensity of the allergenic load and the concentration of pollen.18 Second, climate change could increase the level of allergenic pollens and allergic symptoms associated with exposure, including impacts on pollen amount, and pollen season.2,3 Finally, CO2 concentration and warmer temperature increase pollen concentration. And increased CO2 might alter the allergenicity, or allergen concentration of the pollen, and symptom severity.11,12
The pollen season was expanded and the concentration of pollens and sensitization rate to pollens were increased from year to year in the present study. Sensitization to allergens is an important step in the development of allergic diseases. A longer pollen season and increased pollen amount could increase the opportunity of human exposure to pollen as aeroallergens with consequences for increased allergic sensitization in childhood.17,19
There have been a few epidemiological and experimental studies on the relationship between allergic diseases and environmental and climate changes.20,21 An epidemiological study presented sensitization rate to tree pollens was significantly increased to 36.4% in the 2010s from 19.0% in the 1990s and 8.8% in the 1980s in Korea.22 The nationwide prevalence of “diagnosis of allergic rhinitis, ever” was reported 15.5%, 20.4%, and 28.5% in 1995, 2000, and 2006, respectively in the Korean version of the International Study of Asthma and Allergies in Childhood of Korean children.23,24
Younger children might increase sensitization rate to pollen compared to over 10 years old children with the altered pollen season in this study. A study for children also reported the age for sensitization to weed pollens gradually became younger (4 to 6 year of age, 3.5% in 1997 and 6.2% in 2009; 7 to 9 year of age, 4.2% in 1997 and 6.4% in 2009).17 Difference in exposure rates to allergens with increasing age result in difference in sensitization rates. Although the effects of environmental change on respiratory allergic diseases are not completely understood, these differences in sensitization rates may be implied different living environments such as the playing outdoor and going to school in young children, conversely adolescent stay the indoor due to hard study or playing videogames with recent trend in Korea. It may be useful to manage allergic diseases by continuous investigation and systematical observation of the regional differences and chronological changes of living environments.
Oak, hazel, and alder were shown the sharpest slope to raise sensitization rate among tree species and more related between sensitization rate and pollen season except pine which is no to mild allergenic, during this study. Oak is expected to dominate over other species during the vegetation transition,25 so that its distribution will be broader. By developing phenological models based on weather conditions, one can assess future oak species distribution and length of the pollen season. Hazel pollen is important, resulting in asthma and allergic rhinitis in hazel pollen-allergic individuals, in particular in early spring.26,27 Alder pollen also is a significant cause of asthma, allergic rhinitis, and allergic conjunctivitis, in particular in Spring and in conjunction with birch and hazel pollen.28,29 In patients with poly-sensitization to alder, hazel, and oak pollen who were cross-reactive, initial symptoms occurred as early as February, with abrupt exacerbation in March to May.
The accumulated temperature was the most influential variables to pollination among meteorological variables in this study. Sensitization rate to oak and birch had the highest correlation to accumulated temperature among the subjected trees. Birch has reproductive rhythms of high and low years for the abundance of pollen and subsequent seed and pollination is dependent on the temperature.30
In conclusion, this study demonstrated the positive correlation between the consequent changes of pollen seasons and the raising sensitization rate to major tree pollens in childhood in Seoul metropolitan area for the past 22 years and continuous increase in concentration of pollens due to weather changes were expected. The sensitization rate increased in younger children from year to year. This might imply that climate change impact to alter the pollen season and consequently to increase the sensitization rate to allergic tree pollens in children.